Field of the Invention
This disclosure relates to manufacture of photomasks and, specifically, to manufacture of photomasks for forming optical elements.
Description of the Related Art
A micro lens is known as an optical element for an image pickup apparatus. In the image pickup apparatus, since an incident angle of light incident on a peripheral portion is larger than an incident angle of light incident on a center portion in an image pickup area in which photoelectric conversion elements are arrayed, light condensing characteristics of the micro lens located in the peripheral portion thereof may be lowered in comparison with that in the center portion. Japanese Patent Laid-Open No. 2007-335723 discloses a configuration in which tear drop type micro lenses are provided in a peripheral portion of a hemispherical micro lens located in a center portion of an image pickup area to reduce a loss of light condensing characteristics of the micro lens in the peripheral portion thereof.
Japanese Patent Laid-Open No. 2005-258349 discloses a method of forming a micro lens having a given shape by using a photomask having an area coverage modulation.
When forming a micro lens array (optical element array) having a tear drop shape, which is not a hemispherical shape as disclosed in Japanese Patent Laid-Open No. 2007-335723, using the method disclosed in Japanese Patent Laid-Open No. 2005-258349 is conceivable. However, the present inventors have found that the optical elements having a desired shape cannot be obtained by the method disclosed in Japanese Patent Laid-Open No. 2005-258349 at portions where the optical elements having a non-hemispherical shape abut each other.
Accordingly, this disclosure is intended to provide a photomask which contributes to achieving an optical element having a desired shape and a method of manufacturing an optical element array.